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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Sat 02, 2017 9:44 pm 
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Location: Wood River, Ill.
Well, it is about time that I revived this thread! I did receive the new output transformer that I posted above. Just for information, I got it within the quoted 6 week lead time, and it looks to be very well made and was packed well.

The blue color, while nice, just didn't go well with the black & gold of the Heathkit! I removed the bells and painted semi-gloss black, to match the other iron on the chassis. For mounting, I fabricated an aluminum mounting plate, 1/4" thick, with #8-32 threaded holes to match to the original chassis hole pattern and the new transformer mount. It fit perfectly, and is completely reversible, with no chassis damages or mods. I painted to match the transformer. I do have a detail drawing of the adapter, if anyone has an interest in it.

I wired the new transformer into the amplifier in the original wiring configuration. When I powered up, with the preamp connected, I am hearing an almost inaudible high pitched tone thru the speaker. I connected the scope to the speaker output and can see the highly distorted tone. If I disconnect the pre-amp, the tone is much lower and louder. But I shut down quickly before I got any kind of data off the scope.

It is going into some kind of oscillation from the feed back circuit. Disconnecting the 16 ohm lead from the output transformer, the oscillation goes away. It is as quiet as a mouse, with or without the pre-amp attached. I might add that when it was in oscillation, I could not balance the output tubes. When the feed back was disconnected, it balanced just fine.

I had to shut it down for the day, so have not been able to go any farther, with any scope measurements. The old transformer did have an internal zobel, I think. I do not believe the new transformer has one. Do I need to compensate for it in the new install? I am assuming that I will have to make some changes to the external zobel, and/or the feed back to the input?

Please excuse my ignorance, but am learning as I go.

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Sun 03, 2017 1:01 am 
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Is it possible that you have the two plate leads reversed?

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Sun 03, 2017 1:11 am 
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Hi Don,
It is possible, but I don't think so, unless they reversed the leads when they made the transformer. It is something to check, though. Thanks for the suggestion.

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Tue 05, 2017 6:32 pm 
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Thanks to both Mark and Dan for suggesting that I might have a phasing issue. I first tried reversing the output from the drivers into the KT66 finals. This did eliminate the oscillation at the output. So with that, I reconnected the drive back to the final grids, per the stock wiring diagram. Then reversed the output transformer leads on the KT66 plate and screen socket connections. Testing here revealed no oscillation with the feedback connected.

Now, the problem is very low output. I think the KT66s are good, as I checked them on the TV7 as 6L6s. No shorts and good emissions. I also tried two known good 6L6 in the chassis with the same result as the KT66s. I have also tried substituting in some known good 12AU7 drivers and pre-amp tubes, with no change in output.

With just a cursory voltage check out on the plate and screen voltages on the KT66, they are low. Output from the B+ is low, (how low? I didn't record, my bad!) but not within 10%! The pre-amp voltages aren't right either, but without having valid numbers here, I am not going to guess.

Something is pulling down the B+, but I am not really sure what, at this point. Any suggestions, with my vague tests, at this point, would be helpful.

Thanks for looking.

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Tue 05, 2017 11:18 pm 
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Hi Chris ,

The original power transfomers are notorious for going bad in those amps , pull the rectifier tube and measure the AC at Pins 4 and 6 to ground .

Also what is the Bias voltage when you have it up and running .

If you need help PM me and will come over and look at it

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Thu 07, 2017 2:41 am 
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Hi Mark,
OK, I finally recorded some voltages, and wrote them down, this time


Quote:
The original power transfomers are notorious for going bad in those amps , pull the rectifier tube and measure the AC at Pins 4 and 6 to ground .With the rectifier removed I measured a voltage of 504 VDC to chassis ground at each plate.


Quote:
Also what is the Bias voltage when you have it up and running .
[b]With the rectifier reinstalled I measured 6.4 VDC on grid 1 of V3. The cathode is measuring 52.9. It should measure 8.3 VDC. on V4 grid 1, I measure 8.9 VDC where it should read 6.7 VDC. The cathode here measured 52.9 here too. /b]

The plate of the first stage, V1 is measuring high at 110 VDC, where it should measure 88. The grid of the second half of V1 is also measuring the 110 VDC, of course. The plate on this second half of V1 is measuring low at 243 VDC where I should see 280. The cathode of this side of V1 is also high at 113 VDC where it should see 100.

The voltages on V2 are normal, or at least very close. It is the only stage that is relatively normal. I have gone thru all connections and components in the V1 stage circuit, and all measure well within spec. I have double checked the feed back components and connections to the 16 ohm tap of the output transformer.

This leaves the KT66s, where the plate voltages are running low, at 457 on V3 (SB 480) and 466 on V4 (SB 475). The voltage on pins 2 and 8 of the rectifier is running 20V low at 490 where it should be 510.

I am still in the process of double checking components and for wiring errors.

Mark, I'll be sending you a PM here shortly.

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Thu 07, 2017 4:09 am 
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Pull V2 and measure the plate voltage of V3 and V4 again , what are the cathode voltages on V3 And V4 on pin 8

If the voltages are normal the amp most likely is oscillating a high frequency .

Check the zobel on VI , the 270 mmf and the 4,7K resistor .

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Fri 08, 2017 2:27 am 
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Hi Mark,

Per your suggestions; I pulled V2 and took measurements on the plates and cathodes of V3 and V4. Here is what I found:

V3 Pin3 (P): 464VDC (should be 480)
V3 Pin 8 (K):53.5 VDC (should be 50)

V4 Pin3 (P): 465 VDC (should be 475)
V4 Pin 8 (K): 53.5 VDC (should be 50)

As you can see, it didn't change things much.

I have checked and rechecked the 240 pF and 4.7K output zobel several times, and all appear good.

Thinking about what you said about a high freq. oscillation, shouldn't I see something on the scope if that were present?

One interesting thing that I found, tonight, when dissecting the old transformer, was two 139 pF mica caps, one from the 40% tap to the plate tap on each side of the center tap of the old output transformer. I am assuming this is the internal zobel you were talking about, before. As far as I know, the new transformer does not have anything like this installed. Do I need to make up for this missing cap in my install of the new transformer? I thought about placing a 150 pF cap across the screen and plate tube terminals to emulate this missing cap. I thought I would run it by you first.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Fri 08, 2017 3:28 am 
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Well with V2 out it is not oscillating , Most likely there is too much feed back for the new transfomer .

The new transfomer will be stable with out the two caps on the ultralinear windings .

The 470 ohm resistor on pin 3 of V1 should be changed to two 240 ohm ohm resistors in series with a 25 uf cap from the center point of the two resistors to ground . This will lower the feed back and the gain should go up enough that a proper feedback loop can be determined .

Will talk about that over the coffee .

Mark

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Sep Sat 30, 2017 8:32 am 
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Any progress?


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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Oct Mon 02, 2017 1:05 pm 
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:cry: Not any real progress. It is starting to look like I may have gotten a bad transformer, but still checking it out. Wiring has been rechecked many times. Tubes replaced, rectifier replaced, all with known good tubes. No joy! I am getting low output, which distorts as the input signal increases beyond 1Vpp. Operating the outputs in triode mode results in little, if any improvement. I may find out more this week.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Oct Fri 06, 2017 6:36 pm 
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Progress, at last!
With a lot of help, coaching, and loan of a transformer from Battradio, I tacked in a loaned transformer from Mark. Now I get an undistorted, strong signal out of the amplifier. It is looking like the replacement transformer is the culprit. :)

More to follow.

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Oct Fri 06, 2017 7:15 pm 
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I have the edcor sitting on my desk and it ohms good , but it only reads 56 MH plate to plate and should read more like 15 H .

Have three other output transfomers and all read 15 H to 17 H .

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Oct Mon 09, 2017 10:18 pm 
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battradio@ wrote:
I have the edcor sitting on my desk and it ohms good , but it only reads 56 MH plate to plate and should read more like 15 H . Have three other output transformers and all read 15 H to 17 H .
I don't understand your "56 MH". IIRC, the original Wiliamson amplifier spec called for >100H plate-to-plate OPT inductance, needed to mitigate a tendency to VLF oscillation caused by OPT phase lead at VLF. Your numbers seem very low. Am I missing something? Need some research...
Cheers,
Roger

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Oct Mon 09, 2017 10:55 pm 
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No your not missing anything at all , the transfomer is defective and will be having a nice conversation with Edcors quality control tomorrow .

I have put a signal in the 8 ohm tap and it has the correct turns ratio , with a 7k load on the plate leads . but when DC is applied to the primary it malfunctions .

Some transfomers have interleaving in the winding to mitigate the capacitance in between the windings , something has gone wrong there .

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Mar Wed 07, 2018 2:29 pm 
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I dawned on me, lately, that I have not provided any updates on this project recently. With a lot of help and guidance from Mark, we narrowed the problem down to a bad replacement transformer. As it turned out the transformer primary had an internal short. It took a while, but the manufacturer replaced the transformer.
Due to having other projects on the bench, it was several months before I finally got around to installing the new transformer. I would like to report the new transformer and completed amplifier performs as it should. It sounds great! At least to my ears, anyway.
It sits on a shelf, though, and I will be looking for another W-5M to match it. I have been too busy with other projects to do much more with it right now. I just thought that I owed it to the people who helped out to update this thread.

Thanks to all.

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Mar Sat 10, 2018 3:51 am 
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Chris,

Thanks for the update on your progress. I stumbled upon a W-5M last week (and this thread) and it too suffers from an open 16309 OT, so this thread is helpful in my pursuit of bringing mine back to life.

In the end, using the aftermarket transformer, did you modify the original circuit to compensate for anything or did you leave all the component values the same? I think I remember reading that the Peerless has 50% UL taps, while your replacement has 40% taps. Is this difference anything to be concerned about, or would the circuit need to be modified?

I've been talking to Heyboer, which happens to be local to me, and they may be able to wind a new OT that will fit into the original Peerless can. I may go this route for now as I don't expect to locate an original Peerless anytime soon. Here's a photo of my unpotted 16309 for anyone interested: http://bulbcollector.com/photos/peerless2.jpg

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Mar Sat 10, 2018 9:46 pm 
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ImageImage

https://i.imgur.com/l78OPZP.jpg


I put a pair of pioneer transfomers in the peerless cans to use on an amp im building .

I you pot your own transfomer use casting wax , it has a melting point of over 165 degrees F . It is less messy than tar and easier to use and doesn't smell bad like tar .

Mark

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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Mar Sun 11, 2018 3:40 am 
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Hi Tim,

I ended up using the Edcor transformer, as replaced by the manufacturer. I tried various changes recommended by Mark (batradio@) when I was troubleshooting and trying to make the original Edcor work. In the end the only modification that I kept was to replace the 270 ohm resistors between the screen taps and the screens of the KT-66s with 1K resistors. Otherwise, it is original to the schematic from Heathkit.

The Edcor transformer would not fit the chassis without modification of the chassis. So I made an aluminum plate, 1/4” thick that allowed me to mount the Edcor to the chassis without any modifications. It is easily reversible, should another Peerless transformer come along. Mark’s recommendation for potting your rewound transformer sounds like a good idea, if you are going to pot it. (Personally the thought of potting a transformer, or anything else for that matter, is not very appealing to me. Especially after having to remove it from a few old Philcos, RCAs, and Atwater Kent power supplies.)

Just curious, are you going to use the original core?

Best of luck!

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Chris
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 Post subject: Re: Heathkit W-5M amplifier rebuild
PostPosted: Mar Mon 12, 2018 2:58 am 
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Mark & Chris, thanks for the advice. Heyboer is willing to reverse engineer the original 16309 and build it back up using the original laminations and by cloning the windings, but it's very costly. At this point, being my first vacuum tube amp, I'm leaning towards re-potting using a new Heyboer OT of the same specs that will fit inside the can, and I'll take Mark's advice and use casting wax. Un-potting the old tar wasn't bad at all using a hairdryer on the outside of the can. After a few minutes of heating, the whole thing came out rather easily with little mess and no damage to the original finish.

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